For example, as described in Patent Document 1, when industrially producing trimethylolpropane (hereinafter also referred to as “TMP”) by aldol condensation and crossed Cannizzaro reaction of n-butyl aldehyde (hereinafter also referred to as “NBD”) and formaldehyde in the presence of a base catalyst, ditrimethylolpropane (hereinafter also referred to as “di-TMP”), tritrimethylolpropane (hereinafter also referred to as “tri-TMP”) and bistrimethylolpropane (hereinafter also referred to as “bis-TMP”) are by-produced, and the di-TMP is recovered from a high-boiling mixture thereof.
That is, as described in Patent Document 2, a TMP extract (crude TMP) containing substantially no sodium formate is obtained by extracting a reaction product solution obtained by the reaction of NBD and formaldehyde with a solvent after condensing or without condensing the reaction product solution.
Also, in Patent Document 3, there has been proposed the method in which 2-ethyl-2-propenal (hereinafter also referred to as “ECR”) produced in the above reaction is recovered and reacted with TMP, ECR and formaldehyde to increase an amount of di-TMP produced.
To recover di-TMP from the crude di-TMP as a distillation still residue obtained upon production of TMP, there have been proposed crystallization using ethyl acetate (Patent Document 4), crystallization using water as a solvent in the presence of sodium formate (Patent Document 5), crystallization using 1,4-dioxane as a solvent (Patent Document 6), crystallization using water as a solvent after thin-film distillation (Patent Document 7), crystallization of di-TMP under specific conditions by adding an organic solvent to crude di-TMP (Patent Document 8), use of a water-insoluble or slightly water-soluble organic solvent having a di-TMP partition coefficient of 1 or more as a reaction solvent (Patent Document 9), and the like.
These methods for recovering di-TMP from crude di-TMP have the following problems.
(1) When using water as a solvent in the recrystallization from the crude di-TMP as described in Patent Documents 5 and 7, it is not possible to prevent the resulting crystals from being stained with coloring components. In addition, a mother liquor separated from the crystals is a waste water containing organic substances at a high concentration. Therefore, huge expenditure will be need for purification treatment of such a waste water. Also, huge expenditure of energy will be needed when subjecting the mother liquor to distillation to separate and reuse water contained therein. A combustion treatment of the waster water is also undesirable since such a treatment requires a very large amount of a secondary fuel.
(2) In the crystallization using an organic solvent such as ethyl acetate as described in Patent Document 4, it is not possible to obtain a high-purity di-TMP. In order to increase a purity of the obtained di-TMP, the crystallization procedure must be repeated, resulting in poor yield of di-TMP. In the crystallization using 1,4-dioxane as described in Patent Document 6, it is possible to obtain a high-purity di-TMP only by a single operation of the crystallization. However, the 1,4-dioxane is harmful and forms an explosive peroxide by reacting with oxygen in air. When recovering 1,4-dioxane by distillation to reuse the mother liquor separated after the crystallization, there is a risk of explosion thereof. Therefore, in the industrial procedure, it is desirable to avoid the use of 1,4-dioxane.
(3) When conducting the crystallization under the conditions as described in Patent Document 8, it is possible to efficiently separate and remove bis-TMP as an impurity from the crude di-TMP. However, in the crystallization procedure, it is required to render the reaction solution uniform before initiating the crystallization. For this reason, when the concentration of di-TMP in the crude di-TMP as a raw material is high, the use of a large amount of an organic solvent is required to prepare the uniform solution, so that an amount of a mother liquor produced after filtration of the reaction solution is increased, which tends to result in problems such as increased amount of a waste water, need of a very large amount of energy for recovery of the solvent, and low recovery rate owing to increase in amount of di-TMP distributed in the mother liquor.
(4) When it is intended to efficiently synthesize di-TMP in such a manner as described in Patent Document 9, a large amount of tri-TMP tends to be by-produced. In general, a large amount of bis-TMP is contained in the crude di-TMP. In Patent Document 7, the proportion of a content of bis-TMP to a content of di-TMP in the crude di-TMP is from 30 to 100%. Thus, when a large amount of tri-TMP or bis-TMP is contained in the raw material to be crystallized, these impurity compounds tend to be rapidly crystallized for a short period of time, so that growth of crystals of di-TMP tends to be inhibited, and a particle size of the crystals of di-TMP tends to become very fine. A creamy slurry containing a large amount of the fine crystals is hardly subjected to solid-liquid separation and therefore is difficult to handle especially in an industrial scale. Thus, it is not possible to obtain a high-purity di-TMP in an industrial manner.